A new isolation procedure of nitrate from freshwater for nitrogen and oxygen isotope analysis

The nitrogen (δ¹⁵N) and oxygen isotope (δ¹⁸O) analysis of nitrate (NO₃^–) from aqueous samples can be used to determine nitrate sources and to study N transformation processes. For these purposes, several methods have been developed; however, none of them allows an accurate, fast and inexpensive analysis. Here, we present a new simple method for the isolation of nitrate, which is based on the different solubilities of inorganic salts in an acetone/hexane/water mixture. In this solvent, all major nitrate salts are soluble, whereas all other oxygen‐bearing compounds such as most inorganic carbonates, sulfates, and phosphates are not. Nitrate is first concentrated by freeze‐drying, dissolved in the ternary solvent and separated from insoluble compounds by centrifugation. Anhydrous barium nitrate is then precipitated in the supernatant solution by adding barium iodide. For δ¹⁸O analysis, dried Ba(NO₃)₂ samples are directly reduced in a high‐temperature conversion system to CO and measured on‐line using isotope ratio mass spectrometry (IRMS). For δ¹⁵N analysis, samples are combusted in an elemental analyzer (EA) coupled to an IRMS system. The method has been tested down to 20 µmol NO₃^– with a reproducibility (1SD) of 0.1‰ for nitrogen and 0.2–0.4‰ for oxygen isotopes. For nitrogen we observed a small consistent ¹⁵N enrichment of +0.2‰, probably due to an incomplete precipitation process and, for oxygen, a correction for the incorporation of water in the precipitated Ba(NO₃)₂ has to be applied. Apart from being robust, this method is highly efficient and low in cost. Copyright © 2011 John Wiley & Sons, Ltd.     

Study of nitrogen isotope-selective two-stage IR + UV dissociation of ammonia molecules

The one-photon IR excitation and subsequent UV dissociation of ammonia molecules selective with respect to nitrogen isotopes were studied. The selectivity of vibrational excitation is achieved by tuning CO₂ laser radiation to resonance with ¹⁴NH₃ or ¹⁵NH₃ molecules. The dependences of the yield of dissociation for each isotopic component and the selectivity on the buffer gas (N₂, O₂, Ar) pressure, the partial pressure of ammonia, and the time of delay between IR and UV laser pulses were established. At low pressures (67–270 Pa) of the isotopic mixture with a ¹⁵N concentration of 4.8%, the dissociation selectivity for ¹⁵N was 17. The mechanisms responsible for the selectivity of IR + UV-initiated dissociation are discussed. The phenomenological model has been developed that takes into consideration the effect of the interisotopic V-V exchange and V-T relaxation on the formation of the yield and selectivity of the two-stage IR+UV dissociation of ammonia.     

Measurements of nitrogen isotope composition of plants and surface soils along the altitudinal transect of the eastern slope of Mount Gongga in southwest China

The natural abundances of stable nitrogen isotopes in plants and soils have been viewed as recorders that can be used to reconstruct paleoclimate and ecological processes or to indicate the biogeochemical cycle of nitrogen in nature. This study systematically measured the nitrogen isotope composition (δ¹⁵N) in plants and surface soils along an altitudinal transect of elevation range of 1200 to 4500 m on the eastern slope of Mount Gongga in southwest China. The influences of photosynthetic pathways on plant δ¹⁵N as well as the effects of temperature and precipitation on δ¹⁵N altitudinal trends in plants and surface soils are discussed. Across this altitude transect, the δ¹⁵N values of C₃ and C₄ plants on Mount Gongga range between ?9.87‰ and 7.58‰ with a mean value of ?1.33‰, and between ?3.98‰ and 4.38‰ with a mean value of ?0.25‰, respectively. There is an evident δ¹⁵N difference between C₃ plants and C₄ plants. If, however, you only compare C₄ plants with those C₃ plants growing at the same altitudinal range, no significant difference in δ¹⁵N exists between them, suggesting that photosynthetic pathway does not have an influence on the plant δ¹⁵N values. In addition, we found that C₃, C₄ plants and surface soil (0–5 cm depth) all trend significantly towards more negative δ¹⁵N with increasing elevation. Furthermore, this study shows that the mean annual temperature and the mean annual precipitation positively and negatively correlate with δ¹⁵N in C₃ and C₄ plants, respectively. This indicates that precipitation and temperature are the main controlling factors of the δ¹⁵N variation in plants with altitude. We propose that lower δ¹⁵N values of plants and soils at higher altitude should be attributed to lower mineralization and lower net nitrification rates induced by low temperature and abundant rainfall. Copyright © 2010 John Wiley & Sons, Ltd.     

Error assessment of nitrogen and oxygen isotope ratios of nitrate as determined via the bacterial denitrification method

Currently, bacterial denitrification is becoming the accepted method for δ¹⁵N‐ and δ¹⁸O‐NO determination. However, proper correction methods with international references (USGS32, USGS34 and USGS35) are needed. As a consequence, it is important to realize that the corrected isotope values are derived from a combination of several other measurements with associated uncertainties. Therefore, it is necessary to consider the propagated uncertainty on the final isotope value. This study demonstrates how to correctly estimate the uncertainty on corrected δ¹⁵N‐ and δ¹⁸O‐NO values using a first‐order Taylor series approximation. The bacterial denitrification method errors from 33 batches of 561 surface water samples varied from 0.2 to 2.1‰ for δ¹⁵N‐NO and from 0.7 to 2.3‰ for δ¹⁸O‐NO, which is slightly wider than the machine error, which varied from 0.2 to 0.6‰ for δ¹⁵N‐N₂O and from 0.4 to 1.0‰ for δ¹⁸O‐N₂O. The overall uncertainties, which are composed of the machine error and the method error, for the 33 batches ranged from 0.3 to 2.2‰ for δ¹⁵N‐NO and from 0.8 to 2.5‰ for δ¹⁸O‐NO. In addition, the mean corrected δ¹⁵N and δ¹⁸O values of 132 KNO₃‐IWS (internal working standard) measurements were computed as 8.4 ± 1.0‰ and 25.1 ± 2.0‰, which is a slight underestimation for δ¹⁵N and overestimation for δ¹⁸O compared with the accepted values (δ¹⁵N = 9.9 ± 0.3‰ and δ¹⁸O = 24.0 ± 0.3‰). The overall uncertainty of the bacterial denitrification method allows the use of this method for source identification of NO. Copyright © 2010 John Wiley & Sons, Ltd.     

δ15N of soil N and plants in a N‐saturated,subtropical forest of southern China

We investigated the δ¹⁵N profile of N (extractable NH, NO, and organic N (EON)) in the soil of a N‐saturated subtropical forest. The order of δ¹⁵N in the soil was EON > NH > NO. Although the δ¹⁵N of EON had been expected to be similar to that of bulk soil N, it was higher than that of bulk soil N by 5‰. The difference in δ¹⁵N between bulk soil N and EON (Δ¹⁵N_bulk‐EON) was correlated significantly with the soil C/N ratio. This correlation implies that carbon availability, which determines the balance between N assimilation and dissimilation of soil microbes, is responsible for the high δ¹⁵N of EON, as in the case of soil microbial biomass δ¹⁵N. A thorough δ¹⁵N survey of available N (NH, NO, and EON) in the soil profiles from the organic layer to 100 cm depth revealed that the δ¹⁵N of the available N forms did not fully overlap with the δ¹⁵N of plants. This mismatch in δ¹⁵N between that of available N and that of plants reflects apparent isotopic fractionation during N uptake by plants, emphasizing the high N availability in this N‐saturated forest. Copyright © 2010 John Wiley & Sons, Ltd.     

Absorption and translocation of nitrogen in rhizomes of Leymus chinensis

Leymus chinensis is a dominant species in the Inner Mongolia steppe, northern China. Plant growth in northern China grassland is often limited by low soil nitrogen availability. The objective of this study is to investigate whether rhizomes of Leymus chinensis are involved in the contribution of N uptake. The N concentration, ¹⁵N concentration and ¹⁵N proportion in roots, rhizomes and shoots after 48 h exposure of roots (L_root) and rhizomes (L_rhizo) separately and roots and rhizomes together (L_r+r) to 0.1 mM ¹⁵NHNO₃ solution were measured using root‐splitting equipment and stable isotope (¹⁵N) techniques, respectively. The N content and dry mass were not affected by the labeling treatment. In contrast, the ¹⁵N concentration in shoots, rhizomes and roots was significantly increased by the labeling in rhizomes, indicating that the inorganic nitrogen was absorbed via rhizomes from the solution and can be transported to other tissues, with preference to shoots rather than roots. Meanwhile, the absolute N absorption and translocation among compartments were also calculated. The N absorption via rhizomes was much smaller than via roots; however, the uptake efficiency per surface unit via rhizomes was greater than via roots. The capacity and high efficiency to absorb N nutrient via rhizomes enable plants to use transient nutrient supplies in the top soil surface. Copyright © 2011 John Wiley & Sons, Ltd.     

On the resilience of nitrogen assimilation by intact roots under starvation,as revealed by isotopic and metabolomic techniques

The response of root metabolism to variations in carbon source availability is critical for whole‐plant nitrogen (N) assimilation and growth. However, the effect of changes in the carbohydrate input to intact roots is currently not well understood and, for example, both smaller and larger values of root:shoot ratios or root N uptake have been observed so far under elevated CO₂. In addition, previous studies on sugar starvation mainly focused on senescent or excised organs while an increasing body of data suggests that intact roots may behave differently with, for example, little protein remobilization. Here, we investigated the carbon and nitrogen primary metabolism in intact roots of French bean (Phaseolus vulgaris L.) plants maintained under continuous darkness for 4 days. We combined natural isotopic ¹⁵N/¹⁴N measurements, metabolomic and ¹³C‐labeling data and show that intact roots continued nitrate assimilation to glutamate for at least 3 days while the respiration rate decreased. The activity of the tricarboxylic acid cycle diminished so that glutamate synthesis was sustained by the anaplerotic phosphoenolpyruvate carboxylase fixation. Presumably, the pentose phosphate pathway contributed to provide reducing power for nitrate reduction. All the biosynthetic metabolic fluxes were nevertheless down‐regulated and, consequently, the concentration of all amino acids decreased. This is the case of asparagine, strongly suggesting that, as opposed to excised root tips, protein remobilization in intact roots remained very low for 3 days in spite of the restriction of respiratory substrates. Copyright © 2009 John Wiley & Sons, Ltd.     

Reaction of nitrogen dioxide with iron tetraphenylporphyrinate nitro complex containing the trans-1-methylimidazole ligand

Electron and IR spectroscopy, as well as the data on the isotopic analog ¹⁵NO₂, were used for the studies of the reaction of nitrogen dioxide (NO₂) with thin layers of the six-coordinated Fe^III-porphyrin nitro complex with trans-1-methylimidazole ligand Fe(TPP)(1-MeIm)(NO₂) (TPP is the meso-tetraphenylporphyrinate dianion), which is a heme-modelling system. It was shown that, like in the case of the five-coordinated nitrite complex, additional portions of NO₂ promote the nitrite-to-nitrate transformation, accompanied by the evolution of NO. The reaction leads to the formation in the layer of five- and six-coordinated Fe-porphyrin nitrate complexes and a small amount of imidazole complexes Fe^III(TPP)(1-MeIm)₂ with the outersphere nitrate anion. The latter complex became predominate after treatment of this layer with additional portion of 1-methylimidazole.     

A suite of sensitive chemical methods to determine the δ15N of ammonium, nitrate and total dissolved N in soil extracts

Natural ¹⁵N abundances (δ¹⁵N values) of different soil nitrogen pools deliver crucial information on the soil N cycle for the analysis of biogeochemical processes. Here we report on a complete suite of methods for sensitive δ¹⁵N analysis in soil extracts. A combined chemical reaction of vanadium(III) chloride (VCl₃) and sodium azide under acidic conditions is used to convert nitrate into N₂O, which is subsequently analyzed by purge‐and‐trap isotope ratio mass spectrometry (PTIRMS) with a cryo‐focusing unit. Coupled with preparation steps (microdiffusion for collection of ammonium, alkaline persulfate oxidation to convert total dissolved N (TDN) or ammonium into nitrate) this allows the determination of the δ¹⁵N values of nitrate, ammonium and total dissolved N (dissolved organic N, microbial biomass N) in soil extracts with the same basic protocol. The limits of quantification for δ¹⁵N analysis with a precision of 0.5‰ were 12.4 µM for ammonium, 23.7 µM for TDN, 16.5 µM for nitrate and 22.7 µM for nitrite. Copyright © 2010 John Wiley & Sons, Ltd.     

Biochar coupled with contrasting nitrogen sources mediated changes in carbon and nitrogen pools,microbial and enzymatic activity in paddy soil

There is limited information on changes cause by nitrogen (N) fertilizers and biochar (BC) application in soil carbon and nitrogen availability, leaching and microbial activity at different growth stages in rice. This is first comprehensive study conducted in early and late seasons during 2019 to evaluate efficiency of various traditional N fertilizers (i) Urea (ii) Ammonium nitrate and (iii) Ammonium sulfate (315 kg N ha⁻¹) with or without biochar (30 t ha⁻¹). Results illustrated that all N fertilizers sources applied with biochar significantly increased soil organic carbon (SOC) content by an average 48.44% and 50.63%, soil total nitrogen (Nt) by 4.56% and 4.94%, reduction in total nitrogen leaching by 42.63% and 76.16%, while dissolved organic carbon leaching (DOC) augmented by 39.87% and 38.38% than non-applied treatments in early and late season, respectively. Additionally, soil microbial biomass C and N progressively increased with growth stages and was found higher than non-applied treatments in both seasons. Furthermore, combined application of N fertilizers and biochar, facilitated soil N transformation and the net concentration of NH₄⁺–N and NO₃⁻–N was relatively higher than non-charred treatments. Similarly, in both early and late seasons, urease enzyme activity increased by an average 13.52% and 13.55%, β-glucosidase by 15.99% and 19.27% however, catalase activity decreased by 14.58% and 12.38%, correspondingly. Moreover, no significant difference (p < 0.05) was recorded among N fertilizers sources in both seasons.     

External-source contamination during extraction—distillation in isotope-ratio analysis of soil inorganic nitrogen

The sources of contamination introduced during the extraction, distillation and drying phases of isotope-ratio analysis of soil inorganic nitrogen (ammonium and nitrite + nitrate) were identified, and the individual amounts of contaminants were quantified by isotope dilution. The procedure involves addition of internal standard solutions of ¹⁵N-labelled ammonium and nitrite to reagent blanks which are carried through each stage of the analysis at the same time as the test samples. Potassium chloride extractants, filter-papers, distillation reagents and atmospheric ammonia all contributed to dilution of the sample ¹⁵N. Some materials tested contained sufficient contaminants to cause large errors in the determination of sample ¹⁵N abundance. Both the amount and isotopic composition of contaminants can be determined by the isotope-dilution procedure, which permits the measured sample ¹⁵N abundance to be corrected for contamination.     

Environmental isotopes (15N and 18O) in the assessment of groundwater degradation: Aveiro Quaternary aquifer (NW Portugal)

The Aveiro region, situated on the Northwest coast of Portugal is one of the most industrialized areas of Portugal. The shallow Quaternary aquifer system, mainly of detrital sediments of Pleistocene and Holocene ages, is still today an important water resource for this region despite growing evidence of diffuse pollution problems. Isotope techniques have been used to evaluate the aquifer vulnerability to nitrate contamination (δ¹⁸O and δ¹⁵N from NO₃ and δ²H and δ¹⁸O from H₂O). Nitrogen isotopes were used to identify nitrogen sources and assess agriculture, cattle-breeding, urban and industrial contributions to the nitrogen cycle in the Aveiro ecosystem.     

Determination of Nitrogen and Oxygen Isotopes in Nitrates: A Minireview

Nitrogen and oxygen isotope ratios (δ¹⁵N and δ¹⁸O) in nitrates are important in hydrology, oceanography, atmospheric chemistry, and agriculture. This paper reviews current isotope ratio mass spectrometry methods for the determination of nitrogen and oxygen isotopes in nitrates that include graphite combustion, AgNO₃-ion exchange, Ba(NO₃)₂-acetone, two-step chemical conversion, bacterial approaches, and off-axis integral cavity output laser spectroscopy. This paper introduces the principles, processes, advantages, and disadvantages of these procedures. Future studies should focus on the determination of oxygen isotopes. The development of novel spectroscopic or other isotopic methodology may also be new research directions. In addition, in some nitrates, δ¹⁷O is more sensitive than traditional δ¹⁸O and further development for this isotope is of interest. In addition, the determination of δ¹⁷O may be used to more accurately evaluate δ¹⁸O.     

Effect of acidification on elemental and isotopic compositions of sediment organic matter and macro‐invertebrate muscle tissues in food web research

Stable isotope techniques in food web studies often focus on organic carbon in food sources which are subsequently assimilated in the tissue of consumer organisms through diet. The presence of non‐dietary carbonates in bulk samples can affect their δ¹³C values, altering how their results are interpreted. Acidification of samples is a common practice to eliminate any inorganic carbon present prior to analysis. We examined the effects of pre‐analysis acidification on two size fractions of sediment organic matter (SOM) from marine and freshwater wetlands and pure muscle tissue of a common freshwater invertebrate (Cherax destructor). The elemental content and isotopic ratios of carbon and nitrogen were compared between paired samples of acidified and control treatments. Our results showed that acidification does not affect the elemental or isotopic values of freshwater SOM. In the marine environment acidification depleted the δ¹³C and δ¹⁵N values of the fine fraction of saltmarsh and δ¹⁵N values of mangrove fine SOM. Whilst acidification did not change the elemental content of invertebrate muscle tissue, the δ¹³C and δ¹⁵N values were affected. We recommend to researchers considering using acidification techniques on material prepared for stable isotope analysis that a formal assessment of the effect of acidification on their particular sample type should be undertaken. Further detailed investigation to understand the impact of acidification on elemental and isotopic values of organic matter and muscular tissues is required. Copyright © 2010 John Wiley & Sons, Ltd.     

Geographical traceability of wheat and its products using multielement light stable isotopes coupled with chemometrics

The present study was aimed to investigate the variation of stable isotopic ratios of carbon, nitrogen, hydrogen, and oxygen in wheat kernel along with different processed fractions from three geographical origins across 5 years using isotope ratio mass spectrometry (IRMS). Multiway ANOVA revealed significant differences among region, harvest year, processing, and their interactions for all isotopes. The region contributed the major variability in the δ¹³C ‰, δ²H ‰, δ¹⁵N ‰, and δ¹⁸O‰ values of wheat. Variation of δ¹³C ‰, δ¹⁵N ‰, and δ¹⁸O ‰ between wheat whole kernel and its products (break, reduction, noodles, and cooked noodles) were ?0.7‰, and no significant difference was observed, suggesting the reliability of these isotope fingerprints in geographical traceability of wheat‐processed fractions and foods. A significant influence of wheat processing was observed for δ²H values. By applying linear discriminant analysis (LDA) to the whole dataset, the generated model correctly classified over 91% of the samples according to the geographical origin. The application of these parameters will assist in the development of an analytical control procedure that can be utilized to control the mislabeling regarding geographical origin of wheat kernel and its products.     

Mass spectrometric method for the absolute calibration of the intramolecular nitrogen isotope distribution in nitrous oxide

A mass spectrometric method to determine the absolute intramolecular (position-dependent) nitrogen isotope ratios of nitrous oxide (N₂O) has been developed. It is based on the addition of different amounts of doubly labeled ¹⁵N₂O to an N₂O sample of the isotope ratio mass spectrometer reference gas, and subsequent measurement of the relative ion current ratios of species with mass 30, 31, 44, 45, and 46. All relevant quantities are measured by isotope ratio mass spectrometers, which means that the machines inherent high precision of the order of 10^–5 can be fully exploited. External determination of dilution factors with generally lower precision is avoided. The method itself can be implemented within a day, but a calibration of the oxygen and average nitrogen isotope ratios relative to a primary isotopic reference material of known absolute isotopic composition has to be performed separately. The underlying theoretical framework is explored in depth. The effect of interferences due to ¹⁴N¹⁵N¹⁶O and ¹⁵N¹⁴N¹⁶O in the ¹⁵N₂O sample and due to ¹⁵N ₂ ⁺ formation are fully accounted for in the calculation of the final position-dependent nitrogen isotope ratios. Considering all known statistical uncertainties of measured quantities and absolute isotope ratios of primary isotopic reference materials, we achieve an overall uncertainty of 0.9 (1). Using tropospheric N₂O as common reference point for intercomparison purposes, we find a substantially higher relative enrichment of ¹⁵N at the central nitrogen atom over ¹⁵N at the terminal nitrogen atom than measured previously for tropospheric N₂O based on a chemical conversion method: 46.3±1.4 as opposed to 18.7±2.2. However, our method depends critically on the absolute isotope ratios of the primary isotopic reference materials air–N₂ and VSMOW. If they are systematically wrong, our estimates will also necessarily be incorrect.     

Precise and accurate compound-specific carbon and nitrogen isotope analysis of RDX by GC-IRMS

A new analytical method is presented for the compound-specific carbon and nitrogen isotope ratio analysis of a thermo-labile nitramine explosive hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by gas chromatograph coupled to an isotope ratio mass spectrometer (GC-IRMS). Two main approaches were used to minimise thermal decomposition of the compound during gas chromatographic separation: programmed temperature vaporisation (PTV) as an injection technique and a high-temperature ramp rate during the GC run. δ¹⁵N and δ¹³C values of RDX measured by GC-IRMS and elemental analyser (EA)-IRMS were in good agreement within a standard deviation of 0.3‰ and 0.4‰ for nitrogen and carbon, respectively. Application of the method for the isotope analysis of RDX during alkaline hydrolysis at 50°C revealed isotope fractionation factors ε _carbon?=??7.8‰ and ε _nitrogen?=??5.3‰.     

δ 15N measurement of organic and inorganic substances by EA-IRMS: a speciation-dependent procedure

Little attention has been paid so far to the influence of the chemical nature of the substance when measuring δ ¹⁵N by elemental analysis (EA)–isotope ratio mass spectrometry (IRMS). Although the bulk nitrogen isotope analysis of organic material is not to be questioned, literature from different disciplines using IRMS provides hints that the quantitative conversion of nitrate into nitrogen presents difficulties. We observed abnormal series of δ ¹⁵N values of laboratory standards and nitrates. These unexpected results were shown to be related to the tailing of the nitrogen peak of nitrate-containing compounds. A series of experiments were set up to investigate the cause of this phenomenon, using ammonium nitrate (NH₄NO₃) and potassium nitrate (KNO₃) samples, two organic laboratory standards as well as the international secondary reference materials IAEA-N1, IAEA-N2—two ammonium sulphates [(NH₄)₂SO₄]—and IAEA-NO-3, a potassium nitrate. In experiment 1, we used graphite and vanadium pentoxide (V₂O₅) as additives to observe if they could enhance the decomposition (combustion) of nitrates. In experiment 2, we tested another elemental analyser configuration including an additional section of reduced copper in order to see whether or not the tailing could originate from an incomplete reduction process. Finally, we modified several parameters of the method and observed their influence on the peak shape, δ ¹⁵N value and nitrogen content in weight percent of nitrogen of the target substances. We found the best results using mere thermal decomposition in helium, under exclusion of any oxygen. We show that the analytical procedure used for organic samples should not be used for nitrates because of their different chemical nature. We present the best performance given one set of sample introduction parameters for the analysis of nitrates, as well as for the ammonium sulphate IAEA-N1 and IAEA-N2 reference materials. We discuss these results considering the thermochemistry of the substances and the analytical technique itself. The results emphasise the difference in chemical nature of inorganic and organic samples, which necessarily involves distinct thermochemistry when analysed by EA-IRMS. Therefore, they should not be processed using the same analytical procedure. This clearly impacts on the way international secondary reference materials should be used for the calibration of organic laboratory standards.

The effects of freeze/thaw periods and drying methods on isotopic and elemental carbon and nitrogen in marine organisms, raising questions on sample preparation

Stable isotopes are an increasingly important tool in trophic linkage ecological studies. In studies of large marine animals, isotopic sampling is often given secondary priority to sampling for diversity and biomass aspects. Consequently, isotopic samples are frequently collected subsequent to repeated freezing and thawing of animals, and the results of these studies are often based on the assumption that this pre-treatment does not affect the isotopic values. Our study tested this assumption and examined the difference between oven- and freeze-drying on isotopic values and elemental carbon-to-nitrogen (C:N) ratios. The values for δ(15)N and δ(13)C, percentage nitrogen and carbon, and the C:N ratios were determined from the tissues of six marine species, including invertebrates and fish, as (1) fresh samples, (2) samples thawed once, and (3) samples thawed twice. The drying method, thawing treatment and their interaction did significantly affect the δ(15)N and δ(13)C isotope values for all species. Oven-dried samples had slightly higher δ(13)C and δ(15)N values than freeze-dried samples, although not significant in most instances. For most species, oven-drying produced lower carbon and nitrogen percentage than freeze-drying for samples that had been thawed once, but the C:N ratio was unaffected by the drying method. Repeated freezing and thawing did not affect the isotope values, but it did decrease the percentage carbon and nitrogen for both desiccation methods. We recommend drying samples from fresh wherever possible, and careful choice of desiccation method in light of the fact that most lipid models are based on oven-dried samples and oven-drying could cause enrichment of (15)N or (13)C through evaporation of volatile compounds richer in lighter isotopes such as some lipids. Finally, we recommend that further studies on the specific effects of freezing and desiccation on elasmobranchs is needed. Overall we recommend the use of freeze-drying when possible and to use the samples from freshly caught organisms.     

Stabilization of ammonium dinitramide in the liquid phase

The kinetics of accumulation of the main products of thermal decomposition of ammonium dinitramide in the melt was investigated. The isotope composition of nitrogen-containing gases evolved by the decomposition of ¹⁵NH₄N(NO₂)₂ and NH₄ ¹⁵N(NO₂)₂ was found. Easily oxidized salts, amines, amides, iodides, and other compounds soluble in the melt interfere with the liquid-phase decomposition of ammonium dinitramide.